js如何判断一个对象没有任何自己的属性(为空)?

参考原文
假如有两个空对象, 判断他们是否相等, 结果一定是false:

var a = {}
var b = {}
a == b // false

因为跟c语言类似, 这儿判断的是地址而不是内容.
那么怎么知道a是空呢?
有三个方法. 这儿先假设有一个对象obj

var obj = {}

方法1

Object.keys(obj).length === 0

方法2

Object.getOwnPropertyNames(obj).length === 0

方法3, 自己判断

// Speed up calls to hasOwnProperty
var hasOwnProperty = Object.prototype.hasOwnProperty;

function isEmpty(obj) {

    // null and undefined are "empty"
    if (obj == null) return true;

    // Assume if it has a length property with a non-zero value
    // that that property is correct.
    if (obj.length > 0)    return false;
    if (obj.length === 0)  return true;

    // If it isn't an object at this point
    // it is empty, but it can't be anything *but* empty
    // Is it empty?  Depends on your application.
    if (typeof obj !== "object") return true;

    // Otherwise, does it have any properties of its own?
    // Note that this doesn't handle
    // toString and valueOf enumeration bugs in IE < 9
    for (var key in obj) {
        if (hasOwnProperty.call(obj, key)) return false;
    }

    return true;
}

// test it
isEmpty(""), // true
isEmpty(33), // true (arguably could be a TypeError)
isEmpty([]), // true
isEmpty({}), // true
isEmpty({length: 0, custom_property: []}), // true

isEmpty("Hello"), // false
isEmpty([1,2,3]), // false
isEmpty({test: 1}), // false
isEmpty({length: 3, custom_property: [1,2,3]}) // false